Analog filter circuit and adjustment method thereof

Abstract

An analog filter circuit in which filter characteristic deviation can be adjusted with simple circuitry and its adjustment method can be provided. The analog filter circuit includes a low pass filter and a high pass filter and output signals of both filters are input to a comparison and adjustment section from which an adjustment signal is fed back to the low pass and high pass filters and also input to a predetermined-band pass filter having predetermined correlation to the low pass and high pass filters. The low pass and high pass filters are tuned so that the frequency-gain characteristic line in an attenuation band of the low pass filter linearly falls, while that line of the high pass filter linearly rises, both the lines crossing at a reference frequency. When filter characteristic deviation occurs, it is adjusted to compensate gain difference between the output signals produced by filtering the reference signal with a reference frequency and, accordingly, the predetermined-band pass filter is adjusted to its desired filter characteristic.

1. An analog filter circuit, comprising:
 a band pass filter; 
 a low pass filter and a high pass filter, to which an input signal with a reference frequency is input, the low pass filter and the high pass filter having correlation to the band pass filter in filter characteristics and wherein a frequency-gain characteristic line in an attenuation band of the low pass filter linearly falls, while the corresponding line of the high pass filter linearly rises, both the lines crossing at the reference frequency; 
 a pair of DC converters that convert respective filter signals from the low pass filter and the high pass filter to respective DC signals; and 
 a comparison section that feeds back an adjustment signal corresponding to a signal level difference between the respective DC signals to the band pass filter, the low pass filter, and the high pass filter, and wherein the pair of DC converters comprise; 
 an inverter section that inverts the filter signal; 
 a first differential input pair to which the filter signal and a reference signal are input; 
 a second differential input pair to which an inversion signal output from the inverter section and the reference signal are input; and 
 a converter section that outputs the DC signal according to a signal level difference that the filter signal has with respect to the reference signal, obtained by the first differential input pair, and a signal level difference that the inversion signal has with respect to the reference signal, obtained by the second differential input pair, and 
 wherein the adjustment signal compensates a gain difference between the filter signals from the low pass filter and the high pass filter, and accordingly, the filter characteristics in the band pass filter are adjusted. 
 
     
     
       2. The analog filter circuit according to  claim 1 , wherein, in each of the band pass filter, the low pass filter, and the high pass filter, at least one component element determining a filter characteristic has the same structure and characteristics as corresponding elements in the other filters, the filter characteristic depends on a component element's parameter value, equally among the three filters, and the component element's parameter value is variably adjusted by the comparison and adjustment section. 
     
     
       3. The analog filter circuit according to  claim 2 , wherein the band pass filter, the low pass filter, and the high pass filter are passive filters, and wherein the at least one component element includes a variable capacitor element having a variable capacitance value as the component element's parameter value. 
     
     
       4. The analog filter circuit according to  claim 2 , wherein the band pass filter, the low pass filter, and the high pass filter are active filters, and wherein the at least one component element includes a variable capacitor element having a variable capacitance value as the component element's parameter value and/or a trans-conductance amplifier having a variable mutual conductance as the component element's parameter value. 
     
     
       5. A method for adjusting an analog filter circuit comprising a band pass filter and, a low pass filter and a high pass filter having correlation to the band pass filter in filter characteristics and in which a frequency-gain characteristic line in an attenuation band of the low pass filter linearly falls, while the corresponding line of the high pass filter linearly rises, both the lines crossing at a reference frequency, the method comprising the steps of:
 inputting an input signal with the reference frequency to the low pass filter and the high pass filter; 
 inverting a signal level to output an inversion signal for each of the filter signals that are output from the low pass filter and the high pass filter; 
 outputting, for each of the filter signals, a DC signal in accordance with a signal level difference that the filter signal has with respect to a reference signal, and a signal level difference that the inversion signal has with respect to the reference signal; 
 comparing a signal level difference between the respective DC signals output from the filter signals; 
 feeding back a signal to the low pass filter and the high pass filter to shift back the crossing point of the characteristic lines toward a direction to compensate gain difference between the filter signals in accordance with the signal level difference between the respective DC signals; and 
 adjusting the band pass filter in accordance with the feeding back.